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The subprocess module allows you to spawn new processes, connect to their
input/output/error pipes, and obtain their return codes. This module intends to
replace several older modules and functions:

os.system
os.spawn*
os.popen*
popen2.*
commands.*

Information about how this module can be used to replace the older
functions can be found in the subprocess-replacements section.

See also

POSIX users (Linux, BSD, etc.) are strongly encouraged to install
and use the much more recent subprocess32 module instead of the
version included with python 2.7. It is a drop in replacement with
better behavior in many situations.

Run the command described by args. Wait for command to complete, then
return the returncode attribute.

The arguments shown above are merely the most common ones, described below
in Frequently Used Arguments (hence the slightly odd notation in
the abbreviated signature). The full function signature is the same as
that of the Popen constructor - this functions passes all
supplied arguments directly through to that interface.

The arguments shown above are merely the most common ones, described below
in Frequently Used Arguments (hence the slightly odd notation in
the abbreviated signature). The full function signature is the same as
that of the Popen constructor - this functions passes all
supplied arguments directly through to that interface.

The arguments shown above are merely the most common ones, described below
in Frequently Used Arguments (hence the slightly odd notation in
the abbreviated signature). The full function signature is largely the
same as that of the Popen constructor, except that stdout is
not permitted as it is used internally. All other supplied arguments are
passed directly through to the Popen constructor.

To support a wide variety of use cases, the Popen constructor (and
the convenience functions) accept a large number of optional arguments. For
most typical use cases, many of these arguments can be safely left at their
default values. The arguments that are most commonly needed are:

args is required for all calls and should be a string, or a sequence of
program arguments. Providing a sequence of arguments is generally
preferred, as it allows the module to take care of any required escaping
and quoting of arguments (e.g. to permit spaces in file names). If passing
a single string, either shell must be True (see below) or else
the string must simply name the program to be executed without specifying
any arguments.

stdin, stdout and stderr specify the executed program’s standard input,
standard output and standard error file handles, respectively. Valid values
are PIPE, an existing file descriptor (a positive integer), an
existing file object, and None. PIPE indicates that a new pipe
to the child should be created. With the default settings of None, no
redirection will occur; the child’s file handles will be inherited from the
parent. Additionally, stderr can be STDOUT, which indicates that
the stderr data from the child process should be captured into the same file
handle as for stdout.

When stdout or stderr are pipes and universal_newlines is
True then all line endings will be converted to '\n' as described
for the universal newlines'U' mode argument to open().

If shell is True, the specified command will be executed through
the shell. This can be useful if you are using Python primarily for the
enhanced control flow it offers over most system shells and still want
convenient access to other shell features such as shell pipes, filename
wildcards, environment variable expansion, and expansion of ~ to a
user’s home directory. However, note that Python itself offers
implementations of many shell-like features (in particular, glob,
fnmatch, os.walk(), os.path.expandvars(),
os.path.expanduser(), and shutil).

Warning

Executing shell commands that incorporate unsanitized input from an
untrusted source makes a program vulnerable to shell injection,
a serious security flaw which can result in arbitrary command execution.
For this reason, the use of shell=True is strongly discouraged
in cases where the command string is constructed from external input:

>>> fromsubprocessimportcall>>> filename=input("What file would you like to display?\n")What file would you like to display?non_existent; rm -rf / #>>> call("cat "+filename,shell=True)# Uh-oh. This will end badly...

shell=False disables all shell based features, but does not suffer
from this vulnerability; see the Note in the Popen constructor
documentation for helpful hints in getting shell=False to work.

When using shell=True, pipes.quote() can be used to properly
escape whitespace and shell metacharacters in strings that are going to
be used to construct shell commands.

These options, along with all of the other options, are described in more
detail in the Popen constructor documentation.

The underlying process creation and management in this module is handled by
the Popen class. It offers a lot of flexibility so that developers
are able to handle the less common cases not covered by the convenience
functions.

Execute a child program in a new process. On Unix, the class uses
os.execvp()-like behavior to execute the child program. On Windows,
the class uses the Windows CreateProcess() function. The arguments to
Popen are as follows.

args should be a sequence of program arguments or else a single string.
By default, the program to execute is the first item in args if args is
a sequence. If args is a string, the interpretation is
platform-dependent and described below. See the shell and executable
arguments for additional differences from the default behavior. Unless
otherwise stated, it is recommended to pass args as a sequence.

On Unix, if args is a string, the string is interpreted as the name or
path of the program to execute. However, this can only be done if not
passing arguments to the program.

Note

shlex.split() can be useful when determining the correct
tokenization for args, especially in complex cases:

Note in particular that options (such as -input) and arguments (such
as eggs.txt) that are separated by whitespace in the shell go in separate
list elements, while arguments that need quoting or backslash escaping when
used in the shell (such as filenames containing spaces or the echo command
shown above) are single list elements.

The shell argument (which defaults to False) specifies whether to use
the shell as the program to execute. If shell is True, it is
recommended to pass args as a string rather than as a sequence.

On Unix with shell=True, the shell defaults to /bin/sh. If
args is a string, the string specifies the command
to execute through the shell. This means that the string must be
formatted exactly as it would be when typed at the shell prompt. This
includes, for example, quoting or backslash escaping filenames with spaces in
them. If args is a sequence, the first item specifies the command string, and
any additional items will be treated as additional arguments to the shell
itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh','-c',args[0],args[1],...])

On Windows with shell=True, the COMSPEC environment variable
specifies the default shell. The only time you need to specify
shell=True on Windows is when the command you wish to execute is built
into the shell (e.g. dir or copy). You do not need
shell=True to run a batch file or console-based executable.

Warning

Passing shell=True can be a security hazard if combined with
untrusted input. See the warning under Frequently Used Arguments
for details.

bufsize, if given, has the same meaning as the corresponding argument to the
built-in open() function: 0 means unbuffered, 1 means line
buffered, any other positive value means use a buffer of (approximately) that
size. A negative bufsize means to use the system default, which usually means
fully buffered. The default value for bufsize is 0 (unbuffered).

Note

If you experience performance issues, it is recommended that you try to
enable buffering by setting bufsize to either -1 or a large enough
positive value (such as 4096).

The executable argument specifies a replacement program to execute. It
is very seldom needed. When shell=False, executable replaces the
program to execute specified by args. However, the original args is
still passed to the program. Most programs treat the program specified
by args as the command name, which can then be different from the program
actually executed. On Unix, the args name
becomes the display name for the executable in utilities such as
ps. If shell=True, on Unix the executable argument
specifies a replacement shell for the default /bin/sh.

stdin, stdout and stderr specify the executed program’s standard input,
standard output and standard error file handles, respectively. Valid values
are PIPE, an existing file descriptor (a positive integer), an
existing file object, and None. PIPE indicates that a new pipe
to the child should be created. With the default settings of None, no
redirection will occur; the child’s file handles will be inherited from the
parent. Additionally, stderr can be STDOUT, which indicates that
the stderr data from the child process should be captured into the same file
handle as for stdout.

If preexec_fn is set to a callable object, this object will be called in the
child process just before the child is executed. (Unix only)

If close_fds is true, all file descriptors except 0, 1 and
2 will be closed before the child process is executed. (Unix only).
Or, on Windows, if close_fds is true then no handles will be inherited by the
child process. Note that on Windows, you cannot set close_fds to true and
also redirect the standard handles by setting stdin, stdout or stderr.

If cwd is not None, the child’s current directory will be changed to cwd
before it is executed. Note that this directory is not considered when
searching the executable, so you can’t specify the program’s path relative to
cwd.

If env is not None, it must be a mapping that defines the environment
variables for the new process; these are used instead of inheriting the current
process’ environment, which is the default behavior.

Note

If specified, env must provide any variables required
for the program to execute. On Windows, in order to run a
side-by-side assembly the specified envmust include a valid
SystemRoot.

If universal_newlines is True, the file objects stdout and stderr
are opened as text files in universal newlines mode. Lines may be
terminated by any of '\n', the Unix end-of-line convention, '\r',
the old Macintosh convention or '\r\n', the Windows convention. All of
these external representations are seen as '\n' by the Python program.

Note

This feature is only available if Python is built with universal newline
support (the default). Also, the newlines attribute of the file objects
stdout, stdin and stderr are not updated by the
communicate() method.

Exceptions raised in the child process, before the new program has started to
execute, will be re-raised in the parent. Additionally, the exception object
will have one extra attribute called child_traceback, which is a string
containing traceback information from the child’s point of view.

The most common exception raised is OSError. This occurs, for example,
when trying to execute a non-existent file. Applications should prepare for
OSError exceptions.

Unlike some other popen functions, this implementation will never call a
system shell implicitly. This means that all characters, including shell
metacharacters, can safely be passed to child processes. Obviously, if the
shell is invoked explicitly, then it is the application’s responsibility to
ensure that all whitespace and metacharacters are quoted appropriately.

Wait for child process to terminate. Set and return
returncode attribute.

Warning

This will deadlock when using stdout=PIPE and/or
stderr=PIPE and the child process generates enough output to
a pipe such that it blocks waiting for the OS pipe buffer to
accept more data. Use communicate() to avoid that.

Interact with process: Send data to stdin. Read data from stdout and stderr,
until end-of-file is reached. Wait for process to terminate. The optional
input argument should be a string to be sent to the child process, or
None, if no data should be sent to the child.

Note that if you want to send data to the process’s stdin, you need to create
the Popen object with stdin=PIPE. Similarly, to get anything other than
None in the result tuple, you need to give stdout=PIPE and/or
stderr=PIPE too.

Note

The data read is buffered in memory, so do not use this method if the data
size is large or unlimited.

If dwFlags specifies STARTF_USESTDHANDLES, this attribute
is the standard output handle for the process. Otherwise, this attribute
is ignored and the default for standard output is the console window’s
buffer.

If dwFlags specifies STARTF_USESTDHANDLES, this attribute
is the standard error handle for the process. Otherwise, this attribute is
ignored and the default for standard error is the console window’s buffer.

If dwFlags specifies STARTF_USESHOWWINDOW, this attribute
can be any of the values that can be specified in the nCmdShow
parameter for the
ShowWindow
function, except for SW_SHOWDEFAULT. Otherwise, this attribute is
ignored.

SW_HIDE is provided for this attribute. It is used when
Popen is called with shell=True.

In this section, “a becomes b” means that b can be used as a replacement for a.

Note

All “a” functions in this section fail (more or less) silently if the
executed program cannot be found; the “b” replacements raise OSError
instead.

In addition, the replacements using check_output() will fail with a
CalledProcessError if the requested operation produces a non-zero
return code. The output is still available as the
output attribute of the raised exception.

In the following examples, we assume that the relevant functions have already
been imported from the subprocess module.

On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as
the command to execute, in which case arguments will be passed
directly to the program without shell intervention. This usage can be
replaced as follows:

If backslashes immediately precede a double quotation mark,
every pair of backslashes is interpreted as a literal
backslash. If the number of backslashes is odd, the last
backslash escapes the next double quotation mark as
described in rule 3.